The best way isn't always the civil way and Robin knows well enough that sometimes the civil way isn't enough for people like them to really have it sink in, so of course she's going to skip it to get right to the point. Her anger is controlling her actions at the moment, which means things are definitely going to be rough until she calms down. Peter's response to the first slap is a step in the right direction and she hits him again just as hard.

"That's what you get because you should know better. You're my boyfriend," she emphasized with a hard slap. Then as she spoke more, she kept hitting him after each point as if to make sure he got it. "To me, you're just as equal to my husband. You're not just some boytoy I have on the side to fuck whenever I feel like it. If I want to go out on a date with you, it's because I love you and want to spend time with you. I'm not doing it to bribe you into silence and make you more willing to sleep with me."